Taseqite

Taseqite is a rare mineral[1] of the eudialyte group, with formula Na₁₂Sr₃Ca₆Fe₃Zr₃NbSiO(Si₉O₂₇)₂(Si₃O₉)₂(O,OH,H₂O)₃Cl₂.[2][1] The formula given is derived from the original one and shows a separate silicon at the M4 site, basing on the nomenclature of the eudialyte group.[3] Taseqite, khomyakovite and manganokhomyakovite are three group representatives with species-defining strontium, although many other members display strontium diadochy.[1] Both strontium (^N4Sr) and niobium (^M3Nb) are essential in the crystal structure of taseqite.[2] When compared to khomyakovite, taseqite differs in niobium- and chlorine-dominance.[1]

Taseqite
General
Category	Silicate mineral, Cyclosilicate
Formula (repeating unit)	Na12Sr3Ca6Fe3Zr3NbSi25O73)(O,OH,H2O)3Cl2 (original form)
Strunz classification	9.CO.10
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	R3m
Unit cell	a= 14.28, c= 30.02 [Å]; Z = 3
Identification
Color	Dark- to yellowish-brown; lemon yellow
Crystal habit	Thin tablets
Cleavage	{0001}, fair
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5.5
Luster	Vitreous
Streak	Brownish-white
Diaphaneity	Transparent
Density	3.24 g/cm^3 (measured)
Optical properties	Uniaxial
Refractive index	nω=1.64, nε=1.65 (approximated)
References	[1][2]

Occurrence and association

Taseqite's type locality is the Taseq slope located in the Ilimaussaq complex, Greenland - hence its name. At the type locality taseqite occurs in albitite veins, together with aegirine, analcime, catapleiite, ferrobustamite, hemimorphite, pectolite (silicates); ancylite-(La), calcite, dolomite, strontianite (carbonates); fluorapatite, and sphalerite.[2] Taseqite was found also in Odichincha massif in association with nepheline, alkaline feldspar, aegerine and lamprophyllite.[4]

Notes on chemistry

Admixtures in taseqite include potassium and manganese, with traces of yttrium, cerium, hafnium, tantalum, and tin.[2]

The Raman spectra

The Raman spectra of taseqite have features characteristic of other representatives of the eudialyte group. The most complex structure is observed in the range of 100–1200 cm⁻¹. Pronounced peaks are observed at 127 cm⁻¹ (this peak is also present in the spectra of eudialyte and golyshevite) and 190 cm⁻¹; bands at close (but somewhat higher) frequencies were observed in the Raman spectra of eudialyte, manganoeudialyte, golyshevite, ferrokentbrooksite, and aqualite (at 205–207 cm⁻¹) and in the spectra of georgbarsanovite and raslakite (at 213–217 cm⁻¹). Thus, it is reasonable to suggest that the bands at 127 and 190 cm⁻¹ are due to Na–O and Sr–O stretching vibrations, respectively. A superposition of bands of different widths is observed for taseqite in the range of 250–350 cm⁻¹; the intensities of these bands depend strongly on the orientation of the plane of polarization. The band at 270 cm⁻¹ is comparable with that of the band at 272 cm⁻¹, which manifests itself as a peak in the spectrum of aqualite and as a shoulder in the georgbarsanovite spectrum; however, it is absent in the spectra of the other members of the eudialyte group. Other strong bands are observed at 285 and 310–326 cm⁻¹ (the latter group can be put into correspondence with the strong peak observed in the golyshevite and georgbarsanovite spectra). All these bands, having a preferred polarization along the c axis, are most likely due to the out-of-plane bending vibrations of silicon‒oxygen rings. The weaker band at 387 cm⁻¹ coincides with wide peaks in the oneillite and eudialyte spectra, it is observed as a weak peak in the georgbarsanovite spectrum.

In the range of 530–590 cm⁻¹, there is a strong band of complex shape, peaking at 560 cm⁻¹ and having shoulders at 527 and 540 cm⁻¹. The band at 560 cm⁻¹ was interpreted as a manifestation of the vibrations of (SiO₃)_n rings, although the vibrations of Zr–O and Fe–O bonds can also be involved.

The characteristic peak at 605 cm⁻¹ is comparable with the maximum at 612 cm⁻¹ in the Raman spectrum of golyshevite. The wide peak in the vicinity of 700 cm⁻¹ can be compared with that observed at 700–710 cm⁻¹ in the spectra of almost all EGMs, except for aqualite. The absorption peak at 740 cm⁻¹ is typical of many minerals, including oneillite and eudialyte; it is shifted in the spectra of golyshevite (747 cm⁻¹) and georgbarsanovite (751 cm⁻¹). The similar band in the IR spectra of EGMs is due to the bending vibrations of silicon‒oxygen rings, in which electric dipole moment oscillates mainly along the c axis [1]. This is confirmed by the preferred polarization of the Raman band at 740 cm⁻¹ in the direction perpendicular to the c axis.

The frequency range 900–1150 cm⁻¹, corresponds to the Si–O stretching vibrations. The Raman spectrum of taseqite contains a complex band at 930 cm⁻¹ with a shoulder at 900 cm⁻¹ in this range. The bands in the ranges of 1000–1030 and 1070–1130 cm⁻¹, which are due to the vibrations of silicon‒oxygen rings, are assigned to the stretching vibrations of apical Si–O bonds and Si–O–Si bridges, respectively. The region of O–H stretching vibrations contains a weak peak of complex shape at 3632 cm⁻¹, with shoulders at 3660 and 3670 cm⁻¹, and a wide band in the range of 3400–3550 cm⁻¹, which is due to the water molecules forming relatively strong hydrogen bonds.[4]

References

1. Mindat, http://www.mindat.org/min-26453.html
2. Petersen, O.V., Johnsen, O., Gault, R.A., Niedermayr, G., and Grice, J.D., 2004. Taseqite, a new member of the eudialyte group from the Ilimaussaq alkaline complex, South Greenland. Neues Jahrbuch für Mineralogie Monatshefte Jg. 2004(2), 83-96
3. Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785-794
4. Rastsvetaeva, R. K.; Chukanov, N. V.; Zaitsev, V. A.; Aksenov, S. M.; Viktorova, K. A. (May 2018). "Crystal Structure of Cl-Deficient Analogue of Taseqite from Odikhincha Massif". Crystallography Reports. 63 (3): 349–357. doi:10.1134/s1063774518030240. ISSN 1063-7745.